1. Field of the Invention
This invention relates generally to the in vitro production of human blood cells, and more particularly to the use of recombinant human growth and maturation promoting polypeptides to produce clinically useful quantities of mature blood cells from human pluripotent hematopoietic stem cells.
2. Description of the Background Art
Despite advances in blood typing and in testing for the presence of infectious agents in blood, the use of donated human blood for transfusions remains fraught with danger. Even with typing and cross-matching, there continues to be major risks with blood transfusions including febrile or urticarial reactions (1:100), nonfatal hemolytic reactions (1:25,000) and fatal hemolytic reactions (1:1.times.10.sup.6)(Epstein, Increasing the Safety of Blood Transfusions, American Red Cross, 1992, p.1).
The other major risk of modern blood transfusion is the transmission of infectious agents. Currently, the risk of contracting HIV infection from heterologous blood transfusions has been estimated at between 1:40,000 and 1:150,000 (Epstein, 1992, above). The other major, and occasionally fatal, blood-bone infection is hepatitis, with the risk of contracting hepatitis B infections estimated at less than 1:250,000, and the risk of hepatitis C (non-A, non-B) calculated at 1:500-1:3,000 (Epstein 1992, above). Other less common but periodically significant infectious agents include HIV-2, HTLV-1 and HTLV-2 (less than 1:1.times.10.sup.5), CMV (variable), Yersinia enterocolitica (less than 1:1.times.10.sup.6), and rarely Trypanosoma cruz; (Chagas' disease), human parvovirus B19, Borrelia burgdorfei (Lyme disease), Treponema pallidium (syphilis), plasmodium virus and falciparum (malaria), and human herpes virus type 6 (HHV-6), (Epstein, 1992, above).
Because of these risks, there is an important need for safe alternatives for blood transfusions. Native hemoglobin has been chemically modified by various methods in an attempt to create a blood substitute, but thus far such products suffer from a variety of shortcomings, including nephrotoxicity, excessive O.sub.2 affinity due to loss of 2,3-diphosphoglycerol, a short half-life (usually 4-6 hours), rapid dimerization and excretion, and insufficient plasma concentration (Skolnick J. Amer. Med. Assoc. 268:697 (1992); Vigerou et al., Bull. Acad. Natl. Med., 174:947 (1990)).
Human hemoglobin has been packaged in liposomes for administration as neo-erythrocytes, but such products are difficult to sterilize (particularly against viruses such as HIV), they exhibit a short half-life because they are rapidly cleared by the reticuloendothelial system, and significantly suppress the immune system, thereby predisposing recipients to an increased infection rate (Djordjerich et al, Crit. Rev. Ther. Carrier Syst., 6:131 (1989)).
Perfluorochemicals, (e.g., Fluosol-DA) have been tested as hemoglobin substitutes, but these perfluorocarbons contain a potentially toxic surfactant (Pluronic F-68), they must be stored frozen, and, due to their insolubility, require emulsification. In addition, these fluids require oxygen-enriched (potentially toxic) air for proper oxygen delivery, as well as frequent administration due to a short half-life (Skolnick, 1992 above; Vigeron et al, 1990, above).
It is clear that, despite these efforts, an effective and safe blood substitute is still not available. The Applicant has determined that an attractive alternate approach is, not to develop a substitute blood, but rather to produce clinically useful amounts of natural, mature, differentiated, universally compatible human blood cells under conditions such that the major risks from blood-borne infectious agents and transfusion reactions are absent to insubstantial. Such an approach has been invented, and is described below.